1. Field of the Invention
The present invention relates to a position detection apparatus which detects an absolute position of an object.
2. Description of the Related Art
Previously, an incremental encoder and an absolute encoder as position detection apparatuses have been known. The incremental encoder can be easily configured, but there are disadvantages that position information or angle information is lost and an error is accumulated due to an exogenous noise when an electric power is turned off. On the other hand, the absolute encoder typically has advantages that an error is not accumulated with high accuracy and that it is unnecessary to move to a reference position (a home position) even when the electric power is turned off.
Japanese Patent Laid-open No. S61-108914 discloses an absolute encoder that detects an absolute position. In the configuration of Japanese Patent Laid-open No. S61-108914, an upper-level position signal which has a period rougher than a period of an original signal is generated based on a difference of phases obtained from two scales having periods different from each other. In addition, based on the generated upper-level position signal, a position of a periodic signal finer (shorter) than the upper-level position signal within a period of the upper-level position signal is specified to be converted into a position signal with high accuracy.
However, in the configuration of Japanese Patent Laid-open No. S61-108914, the upper-level position signal is generated by using the difference between the two phase signals which are generated by two pairs of sine waves and cosine waves. Therefore, an error contained in the upper-level position signal is greater than an error contained in the original phase signal. When the position of the periodic signal finer (shorter) than the upper-level position signal within the period of the upper-level position signal is specified by using the upper-level position signal, an error detection may easily occur due to the error of the upper-level position signal. Therefore, a detection region (a detection stroke) is limited.